Light Exposure and the Role it Plays in Sleep

The winter is here, days are shorter, and we experience much more of our afternoons in darkness. You have heard of seasonal depression before and how some of us are immensely affected by light, but how significant is this really? What does natural light exposure really do to our natural sleep/wake cycles and mood?

Light exposure plays a crucial role in regulating our 24-hour internal system, the circadian rhythm. As light enters the eye, it is sent through photoreceptors that turn light into electrical signals. There are two major types of photoreceptors—rods and cones. Rods absorb most wavelengths and are very sensitive. Rods are used to see in low-light conditions. On the other hand, cones have more variety and absorb long, medium, or short wavelengths of light since they are made up of different photopigments. These electrical signals then travel to the optic nerve in the brain and create the images that we see. Light has many types of wavelengths, some that are not even visible to us at all, pretty cool right? Light is converted into biological signals by the absorption of this energy.

Light affects the circadian rhythm based on the time of day the light exposure occurs. Exposure to morning light advances the circadian clock. According to Blume and colleagues, exposure to natural daylight has been shown to advance the timing of sleep to earlier in the day, affect duration, and improve the quality of sleep. Our sleep-wake cycle used to be easily regulated when the only light exposure present was natural daylight. However, there is now a significant amount of artificial light present in our everyday life. This artificial light can play a large role in our circadian rhythm and sleep/wake cycles. Exposure to artificial light can delay our circadian clock and in turn, delay sleep. There have been numerous studies that have shown that smartphone use before bed can be associated with poor sleep quality and increased difficulty falling asleep. One study by Mason and colleagues showed that light exposure during sleep increased insulin resistance in the morning, decreased heart rate variability, and increased nighttime heart rate.

Beyond impacting the circadian rhythm, light exposure also affects melatonin production. Melatonin is a hormone that is made by our pineal gland. Melatonin will start to increase in the evening and continues to rise as you are asleep. It is important to note that light inhibits melatonin production. Knowing this, it is evident that during the winter months, there is less light exposure, hence more melatonin release. This hormone directly connects us to our environment and the calendar year which is pretty neat if you really think about it. Beyond making us drowsy and ready for bed, melatonin also has many other effects throughout the body. It can affect our bone mass by increasing the production of osteoblasts, which promote bone production. Melatonin also has a great effect on the central nervous system in regard to how drowsy we feel. There are also protective effects of melatonin since it is also a potent antioxidant.

Knowing how much of an impact natural light as well as artificial light has on the sleep/ wake cycle what can you then do to ensure the best night's sleep possible?

- Be mindful of artificial light exposure in the evening, prior to bedtime, or in the middle of the night.

-Make the room as dark as possible.

-Try to get natural light exposure early in the day to stay in tune with our natural sleep/wake schedules

-If you need to be awake late at night, red light that is dimmed is the best choice because it will not inhibit melatonin production

-Shut off electronics at least 1-2 hours before bed

Sources:

https://www.sleepfoundation.org/bedroom-environment/light-and-sleep

https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/how-eyes-work#:~:text=When%20light%20hits%20the%20retina,into%20the%20images%20you%20see.

https://www.pnas.org/doi/10.1073/pnas.2113290119

https://my.clevelandclinic.org/health/articles/23411-melatonin

https://www.youtube.com/watch?v=UF0nqolsNZc

https://pubmed.ncbi.nlm.nih.gov/27500468/